The finding supports a broad scientific consensus that the mercury-containing preservative thimerosal does not cause autism, and will likely disappoint parents who are convinced otherwise.

Reporting from Washington and Los Angeles — The federal "vaccines court" ruled Friday in three separate cases that the mercury-containing preservative thimerosal does not cause autism, a finding that supports the broad scientific consensus on the matter but that greatly disappointed parents who are convinced that their child's illness was caused by vaccines.

The court had ruled 13 months ago that a combination of the measles-mumps-rubella vaccine, commonly known as the MMR vaccine, and thimerosal does not cause the disorder, so the new ruling may finally close the bulk of litigation on the matter. The earlier ruling has been appealed to the U.S. Court of Appeals, and this one most likely will be also, but most experts think the court will uphold the decision.

A claim that the MMR vaccine alone causes autism has been withdrawn by parents.

More than 5,300 parents had filed claims with the vaccines court, a branch of the U.S. Court of Federal Claims, seeking damages because they believed their children had developed autism as a result of vaccinations. And they reacted bitterly to Friday's ruling.

"Find me another industry where the U.S. government defends their product in court and funds the science that exonerates them," said J.B. Handley, a founder of Generation Rescue in Sherman Oaks and father of a child with autism. "The average citizen has no hope."

The cases that three judges, called special masters, chose to rule on as test cases were considered among the strongest, so the outlook appears grim for others making the same claim. Each ruled on one case.

Special Master Denise K. Vowell wrote in one of the decisions that "petitioners propose effects from mercury in [vaccines] that do not resemble mercury's known effects in the brain, either behaviorally or at the cellular level. To prevail, they must show that the exquisitely small amounts of mercury in [vaccines] that reach the brain can produce devastating effects that far larger amounts experienced prenatally or postnatally from other sources do not."

She also dismissed claims that some groups of children are unusually susceptible to the effects of mercury. "The only evidence that these children are unusually sensitive is the fact of their [autism] itself."

In a separate ruling, Special Master George L. Hastings wrote: "This case . . . is not a close case. The overall weight of the evidence is overwhelmingly contrary to the petitioners' causation theories."

Commenting on the rulings, Dr. Paul Offit of the Children's Hospital of Philadelphia said in a news conference that the idea that vaccines or thimerosal cause autism "had its day in scientific court and was shown not to hold up. . . . The ruling clearly supported the science, fortunately." Offit, inventor of the rotavirus vaccine and author of five books on the vaccine controversy, is a strong proponent of vaccination and has been vilified by many parents.

Parents and advocacy groups argued that the ruling represents a conspiracy to protect vaccination programs. "The courts won't concede something that will bring down the vaccination program," Handley said.

Vaccine court special masters are protecting the vaccine program at the expense of children harmed by inoculations, said Mary Holland of the Coalition for Vaccine Safety, an umbrella organization of autism groups that says it is focused on improving vaccine safety science.

"I'm sure they sincerely believe they're protecting the public health because they think that if people believed vaccines caused diseases, they would stop vaccinating children," Holland said. "They bend over backwards to not acknowledge vaccine injury."

Largely because of parental fears, thimerosal was removed from all childhood vaccines by 2001, except for multidose vials of influenza vaccine. Despite that action, the prevalence of autism has continued to grow, and it is now thought to affect as many as one in every 100 children, according to the Centers for Disease Control and Prevention.

The vaccine court was established in 1986 because vaccine manufacturers were facing many liability suits that threatened their ability to continue manufacturing the medicines. The court holds no-fault hearings to determine whether a child has been harmed by a vaccine. Compensation comes from a $2.5-billion fund based on a 75-cent surcharge on each dose of vaccine.

The court has made many awards to parents who successfully showed that their children were damaged neurologically or otherwise by vaccinations -- a rare, but nonetheless real event -- but has refused to accept claims that autism is caused by vaccination.

The Supreme Court on Monday agreed to hear an appeal from Pittsburgh parents who want to sue vaccine manufacturer Wyeth directly because their daughter suffered a series of debilitating seizures after being vaccinated. They argue that they cannot get a fair hearing in the vaccine court.